1. Field of the Invention
The present invention relates to a phase locked loop (PLL) apparatus for an orthogonal frequency division multiplexing (OFDM) system having variable channel band and an operating method thereof; and, more particularly, to a PLL apparatus for an OFDM system having variable channel band and an operating method thereof for improving a radio frequency (RF) performance of the OFDM system by enabling a PLL module to have different phase noise characteristics for each of channel bands by changing a loop filter of the PLL module, thereby sustaining orthogonality between subcarriers.
This work was supported by the Information Technology (IT) research and development program of the Korean Ministry of Information and Communication (MIC) and the Korean Institute for Information Technology Advancement (IITA) [2005-S-016-02, “Development of Multimode Base Station”].
2. Description of Related Art
An orthogonal frequency division multiplexing (OFDM) system uses the orthogonality between subcarriers. Therefore, if the orthogonality between subcarriers reduces, the system performance thereof is also deteriorated. That is, the frequency offset and the phase noise of a phase locked loop (PLL) module having an oscillator are main factors to deteriorate the system performance in a view of a radio frequency (RF).
It is impossible to compensate the phase noise once the phase noise is generated although the frequency offset can be compensated by automatically controlling a frequency of a system. Therefore, the phase noise between subcarriers is more important in the OFDM system.
Hereinafter, a phase locked loop (PLL) apparatus of an orthogonal frequency division multiplexing (OFDM) system using a fixed channel band according to the related art will be described with reference to FIGS. 1 to 3.
As shown in FIG. 1, the OFDM system according to the related art has a fixed subcarrier gap because the OFDM system according to the related art uses one fixed channel band. Therefore, the system performance deterioration is prevented by minimizing the phase noise of a PLL module in a subcarrier offset as shown in FIG. 2.
However, the subcarrier gap is variable if the OFDM system according to the related art uses a variable channel band. Therefore, if the OFDM system according to the related art uses a PLL module having constant phase noise characteristics regardless of a channel band as shown in FIG. 1, the orthogonality between subcarriers becomes deteriorated because the phase noise of the PLL module operates as a noise in adjacent subcarrier, thereby deteriorating the system OFDM performance.